The present invention relates in part to a new technical grade thermomechanical pulp, which is characterized by an extremely high bulk characteristic and high resistance to degradation under paper-making conditions, while at the same time being extremely economical to produce and utilize. Because of these significant characteristics, the new pulp finds adventageous uses in a variety of specialty products. Additionally the invention pertains to novel filter media and to the use of such media for filtration, in particular for the filtration of air, fuel, or lube oil.
Thermomechanically produced pulp is a well known product in a general sense. It has been widely used for many years in the production of fiber board products, construction paper, newsprint, and other products. For example, in Asplund, U.S. Pat. No. 2,008,892, it is disclosed that thermomechanically produced pulp is useful for the production of wall board, insulation board and similar products, as well as cardboard. (See also Asplund U.S. Pat. No. 2,047,170.) U.S. Pat. No. 4,219,024, issued to Patience et al, discloses the use of thermomechanically produced pulp, as well as mechanically and semi-chemically produced pulp, in an absorbent article to capture body fluids, comprising a pad made of a mass of said fibers and particles of plastic material fused to the fibers in the pad to increase the integrity thereof.
In a thermomechanical pulping process, wood chips or other lignin-containing materials are generally placed under a steam atmosphere at elevated pressure. After preheating in the pressurized steam atmosphere, the wood chips are progressively introduced between a pair of rotating refiner discs while maintained under steam pressure. Depending upon the pressure of the steam, and the adjustment of the refiner discs, the wood chips are subjected to a controlled degree of abrasion to thereby reduce the chips to fibrous form. The fibers then issue from the peripheral region of the rotating disc refiner for further processing and utilization.
It has been known that under certain pressure/temperature conditions in a disc refiner, and certain energy level utilization during the refining process, there is a heat softening or thermoplasticity of the lignin within the fiber bundles. This brings about a weakening of the so-called middle lamella, such that the bonding force between adjacent fibers is greatly reduced and the individual fibers are easily separated without excessive damage and breakage. These original discoveries date back to the early 1930s and are reflected in, for example, U.S. Pat. No. 2,008,892 and U.S. Pat. No. 2,145,851, both issued to Asplund. Developments in equipment and techniques for the thermomechanical manufacture of pulp have of course continued since the pioneering efforts of Asplund. Examples of more recent efforts are the Shouvlin et al. U.S. Pat. No. 3,773,610, assigned to Bauer Bros. Co., Springfield, Ohio, and the Selander et al. U.S. Pat. No. 4,221,630.
Notwithstanding the more or less continuous development effort over the last 50 years or so in the art of thermomechanical pulp production and the utilization thereof, however, no one has heretofore discovered or recognized the truly exceptional high bulk characteristics of certain types of thermomechanically produced pulp fibers for use in the manufacture of fibrous filter media and the beneficial use of such filter media in processes of filtration.
It has now been discovered, however, that thermomechanically produced pulp fibers, produced under certain controlled conditions as specified herein, result in a pulp and fiber which is nearly a theoretical ideal for use in the manufacture of fibrous filter media and for other end uses wherein the high bulk and other desireable characteristics of the pulp are important. Such thermomechanically produced pulp fibers are far superior to ordinary chemical pulps utilized for these purposes, and indeed superior in many respects to the so-called high performance chemical pulps.
The new pulp is derived by the thermomechanical production in a disc refiner of pulp from lignin-containing source materials having a lignin content of at least about 10%. Under the conditions of the invention, the lignin-containing source material is processed under considerable steam pressure, in a range of about 50 psig to about 120 psig, at temperatures in the range of about 300.degree. F. to about 350.degree. F. Additionally, the disc refiner adjustments are such that, under the specified pressure/temperature conditions, there is an energy utilization in the fiber production of about 8 to about 35 Horse Power Days per Air Dried Ton (HPD/ADT) of the material. These conditions, as will be explained more fully hereinafter, enable the production of a technical grade pulp material which has a uniquely advantageous fiber structure for a variety of paper and board products in which low apparent density is one of the desirable attributes. In particular, the pulp has extraordinary advantages when incorporated in filter media.
Felted nonwoven materials are widely used in the manufacture of filter media, for example, such as automative oil, air and fuel filters. Such filters are presently manufactured in large volume by conventional, wet paper-making processes, although air-laying is also a viable and potentially preferable manufacturing procedure. Ideal materials for filter media are bulky, porous, strong, stiff, resistant to heat and chemical degradation, and insensitive to water. The media also should have a desired average pore size for its intended function. These various desirable attributes can generally be controlled by varying the fiber furnish, the treatment of the fibers prior to forming, the manner in which the web is formed, and the post treatment of the formed web. The post treatment, for example, may include any or all of the following: pressing, drying, binder addition, and corrugation.
Physical characteristics of the fiber are of course highly significant. In this respect, experience indicates that ideal fibers should be relatively strong and stiff. They should also possess some degree of curl, yet be relatively non-bonding. Currently, chemical pulps are widely utilized in the production of fibrous filter media because the chemical pulps are characterized by a greater fiber length, higher individual fiber strength and stiffness, and a relatively large degree of fiber bonding when made into paper. By contrast, conventional ground wood pulps are very short, leading to a filter sheet which lacks desired strength and porosity, among other things.
Unfortunately, the most desirable grades of chemical pulps for use in the production of fibrous filter media are also extremely expensive. One such commercially available chemical pulp, i.e., a mercerized southern pine, forms the basis for a superior type of conventional filter sheet possessing a low level of fiber bonding, along with high porosity and high bulk. However, the chemical process for the manufacture of this pulp is rather comlex, and the yield of pulp is quite low, i.e., about 35% based on oven dry wood. Accordingly, this pulp is very costly. Some of the other chemical pulps, particularly flash dried kraft, are somewhat less costly but are correspondingly less satisfactory on a performance basis, and their use is limited to applications in which the performance specifications of the filter media are less demanding.
In accordance with one aspect of the present invention, a novel and greatly improved filter media is provided, as well as efficient processes of filtration utilizing same. The filter media of the present invention utilizes thermomechanically formed pulp in lieu of some or all of the content of high performance and other chemical pulps otherwise customarily utilized. Nevertheless, not only is there no significant loss in filter performance characteristics, but there is indeed an improvement in some of the more significant filter characteristics as a result of the substitution. Additionally, where the new filter media incorporates thermomechanically formed fibers in lieu of lower performance fibers, such as flash dried krafts, a significant performance benefit is realized in the use of the filter media in filtration processes.
The filters of the present invention are characterized by an exceptionally high dust capacity, high freeness, high void fraction and high bulk, all highly desirable characteristics of filter media. In many respects, the filters of the present invention exceed the performance characteristics of conventional filters using high performance chemical pulps. Moreover, the filters of the present invention are capable of achieving superior performance in a broad spectrum of filtration processes, and in particular, in processes for filtering air, lube oil, fuel or pharmaceuticals.